WASHINGTON — Serious crime was 13% lower in Los Angeles in the first six months of 1994 than it was during the same period last year, while the nation as a whole experienced a much smaller 3% decrease, the FBI reported.

Not only did the decline in Los Angeles outpace that of the rest of the nation, it was notably greater than the average 6% drop recorded for cities of more than 1 million residents.

The semiannual decline nationally followed annual decreases of 2% in 1993 and 3% in 1992.

Federal officials offered no explanation for the trend nationally or locally, and they cautioned against too much optimism.

"Any decline in reported crime is welcome, but the level of crime is still unacceptably high and it must be substantially reduced," said a statement accompanying the FBI report.

That serious crime was down in Los Angeles during the first half of 1994 may be a further indication that the city's annual homicide mark could drop below 1,000 for the first time since 1989.

Citywide, 378 homicides were reported during the first six months of 1994, compared to 532 during the same period last year, according to FBI figures.

Authorities in Los Angeles have attributed the decrease to more active neighborhood crime-prevention efforts, greater police focus on high-crime areas, a truce among some African American street gangs and a Mexican Mafia edict against drive-by shootings, which has cooled tensions among Latino gangs.

Other Los Angeles authorities have speculated that the drop may in part be the result of unusual events in the region, such as the Northridge earthquake in January and the World Cup soccer competition last summer.

While acknowledging the possible contribution of all of those factors, Lt. John Dunkin, a spokesman for the Los Angeles Police Department, echoed the sentiment of federal officials, saying: "The crime rate is still unacceptably high. For example, we'll have 900 or so homicides this year. Certainly we can't relax."

Gerald M. Caplan, a former federal law enforcement official who is dean of the McGeorge School of Law in Sacramento, also offered a note of caution.

"Data collection is not so refined that one can safely predict a trend in this area," Caplan said. "Besides, the figures don't tell everything in view of frightening new crimes like carjackings and gang shootings that affect so many of our neighborhoods."

Demographics may explain some of the modest decline, Caplan and others suggested. They pointed to the shrinking numbers of Americans in the 18-to-26 age group, which accounts for most violent crime.

Dr. Jan Chaiken, an authority on crime figures at the U.S. Justice Department, remarked that "people also are working more to protect themselves, which is why we have seen a decline in burglaries and car thefts."

The FBI divides serious crimes into two categories: crimes of violence, such as murder and rape, and crimes against property, such as burglary and arson. Among the violent crimes measured for the first six months of this year, murder decreased 2%, forcible rape, 6%; robbery, 4%, and aggravated assault, 3%.

In the property crime category, burglary was down 6%, and larceny and motor vehicle theft each declined 2%. Arson showed no change.

The FBI also released its 1993 annual publication, "Crime in the United States." The final figures showed an estimated 14.1 million serious offenses were reported in 1993 to law enforcement agencies across the nation, or 5,483 crimes for every 100,000 inhabitants.

Aggravated assaults accounted for 59% of the violent crimes reported last year. Robberies comprised 34%; forcible rapes, 5%, and murders, 1%. Firearms were the weapon of choice in 32% of all murders, robberies and aggravated assaults.

The FBI said the proportion of violent crimes committed with firearms has increased in recent years. In 1989, firearms were used in 27% of all violent offenses.

An estimated 2.8 million arrests were made in 1993 for all serious offenses, with some arrests representing multiple crimes. Of all those arrested last year for crimes other than traffic offenses, 45% were under the age of 25, and 81% of all arrestees were males.

Elsewhere in the Southland in the first six months of 1994, Pasadena recorded the biggest drop in its overall crime rate--almost 20%. The rate dropped even though Pasadena recorded the same number of homicides--nine--for the first six months of this year compared to the same period last year, according to a Times computer analysis of the FBI figures.

Other communities recording significant reductions in their overall crime rates were Inglewood--despite an increase in homicides from 23 to 26--Thousand Oaks and Garden Grove. Pomona also recorded a significant reduction in its crime rate, notwithstanding 24 homicides for the first six months of 1994, up from 16 for the same period last year.

Thousand Oaks recorded the lowest crime rate for the six-month period among big cities in the region--1,333 per 100,000 people for the first six months of this year, The Times' analysis showed.

Times staff writer Richard Simon in Los Angeles contributed to this story.

14.1 million serious offenses were reported in the United States in 1993. Aggravated assaults: 59% Robberies: 34% Forcible rapes: 5% Murders: 1% Other: 1% Firearms were used in 32% of all serious crimes.